Method of making stretchable unified paper



Patented Mar. 31, 1953 .METHOD F MAKING STRETCHABLE UNIFIED PAPER Waldo 'Kellgren,

Birchwood,

and ilohn W.

Marschall, St. Paul, Minn.,-assignors toll/[innesota Mining a Manufacturing Company, St. Paul, Mimn; a corporation of Delaware No Drawing. Application 'April'26, 1950, 'serial'No. 158.324

4 Claims.

Thi application is a'continuation-in part of our co-pending'applicaticnser. No. 749,874, filed on May 22, l947,-and now abandoned.

This invention relates to a new and useful type of stretchable unified paper; to the method of making, which involves the step of severely calendering stretchable creped'paperfollowed by application of afiber-unifying composition; and to novel adhesive tapes and "other products made from such paper.

By unified paper is'meant paper that has been coated or impregnated with a material which bonds the paper fibers'together so as to increase the tensile strength of the paper and its resistance to internal'splitting or "delamination. By stretchable paper'is'meant paperthat has a special structurepermittingofa substantialelongation (at least before rupture when hand pulled; as distinguished from the slight degree of elongation of ordinary paper'when strongly pulled to the breaking point (due to Slipping of the intermeshed fibers in their fiat lay relation, which is a rupture yielding effect) 'Creped paper is a Well known type of stretchable paper. The wrinkling of the paper permits of elongation'by'hand pulling to a substantial extent'biore slipping and yielding of the fibers occurs preliminary to complete rupture. The creping operation also opens up the fiber structure and decreases the density of the paper, and makes the paper more porous and absorbent. The caliper thickness is considerably increased, due both to the wrinkling and to the decrease in density of the body structure. .Creped paper can be impregnated with waterproofstretchable compositions so .as to result in unified paper that is both stretchable and waterproof.

Unified creped paper of this type is commercially employed on a large scale as backings in pressure-sensitive adhesive masking tapes. The normally tacky and pressure-sensitive adhesive coating causes instantaggressive adhesion on contact with surfaces to which applied by hand,

without the need of moistening or heating, and

the tape can be dry stripped without internal delamination and without leaving an adhesive residue. Such masking tapes, supplied in roll form, are widely used in connection with painting operations (such as lacquer spraying of automobiles). The stretchable characteristic permits easy conformation to curved or irregular surfaces, and permits the tape to be applied so as to define a curved masking edge when desired. The impregnation of the paper-backing prevents .penetration 'by lacquer solvents and the like which 2 would soften the-adhesive and prevent clean removal of the tape.

The present invention makes possible the production of adhesive masking tapes having improved use characteristics, while at the same time materially reducing the cost of manufacture. This highly desirable goal has been realized by structural changes in the tape obtained as'the result of modifying the paper backing structure. The consequence of the change is not-merely to alter the backing but also-to alter the relationship of the adhesive coating to the backing, and thereby to-obtain a new combination having-new and useful properties. Reduction in cost of manufacture is a highly important end in itself but in this case has been accomplished in conjunction with very substantial improvements in the performance characteristics of the product.

While a particularly important accomplishment of the invention thus relates to adhesive masking tapes, the invention also has value in providing a new type-of stretchable unifiedpaper that can be used,in one or'another specific'form, in mak ing other types ofproducts.

This invention has-resulted from our discovery of the unexpectedresults that can be'obtained by subjecting porous creped paper to severe calendering, followed by application of astretchable fiber-unifying composition.

It might well besupposed' that'severe calender-' ing of porous-creped paper would have the effect of greatly reducing the stretch characteristic of the finished masking tape, and would-adversely affect its tensile strength; as compared with masking tape made in the usual way with the same creped paper (not calendered). Especially so since severe calende'ring does'materially reduce both' the stretch and tensile strength values of the crepedpaper per se.

We have discovered, however, that severe calendering of the porous creped paper, sufficient to smooth down the creping wrinklesand to reduce the'caliper thickness by as much as one-fifth or even onefhalf or more, does not have the expected result. Indeed, stretch and tensile strength values for the tape product can be obtained which are if anything somewhat higher than would result if the calendering step had been omitted.

We have further discovered that such calendering of the porous creped paper decreases the weight of .impregnant per unit area which is required for the same efiective unification of the fibers an barrier action to lacquersolvent penetration. We have also discovered that a lower coating weight of pressure-sensitive adhesive can be used without impairing tape adhesion characteristics. The saving in amounts of impregnant and adhesive needed per unit area for the same performance characteristics, results in a cost saving which is a substantial proportion of the total monetary cost of manufacturing the masking tape. In comparison, the cost of calendering the creped paper is trivial.

Each of these changes contributes to the improved performance characteristics of the finished masking tape. Adhesive tape can readily be produced which has a total caliper thickness less than one-half of what it would have if made from the same creped paper (not calendered) with the normal impregnation and adhesive coating. Tape rolls are made smaller in diameter. The improved tape has a softer and more pliant feel and handles better in applying it to objects being taped. Yet there is no sacrifice of stretch and tensile strength values.

A particularly noteworthy advantage is that the tape is flatter and has a thinner edge. Inpainting operations (both spray painting and brush painting) this means that a cleaner and sharper line of demarcation is obtained at the juncture of the painted and masked surface areas. There is less tendency for the fluid paint or lacquer to seep under the tape or to build up on the edge of the tape, but at the same time the paint or lacquer coating is more certain to uniformly cover the surface up to the surface-contacting edge of the adhesive layer.

The invention also permits of using creped paper which would not be regarded as suitable when employed for making masking tape in the customary way. Thus a creped paper having a high degree of creping and stretch and a relatively high caliper thickness, can be calendered down to a reduced caliper thickness and the paper will be relatively fiat. By using this calendered creped paper in place of an uncalendered creped paper of the same caliper thickness as the calendered paper, a finished masking tape can be made which has no greater total caliper thickness and yet has much higher stretch and tensile strength values, and the adhesive coating has a flatter surface.

Thus stronger and stretchier masking tapes can be made without increase of caliper thickness; or substantially thinner and more economical tapes can be made without sacrifice of strength and stretchiness; and the manufacturer thus has greater latitude in designing different types of masking tapes for various customer preferences.

The term porous creped paper is used by us in the strict sense and designates porous paper that has been crinkled, as by crowding a wet adherent sheet on a roll by means of a doctor blade, or by some equivalent operation, so as to produce an irregular wrinkling. The term does not embrace pleated papers or corrugated papers, which are entirely different, and are so recognized in the trade. The rolling down of corrugated paper so as to produce a pleated formation (wherein portions of the paper are folded over flatwisel upon themselves, so that a cross-section of the folded portion shows three layers of paper inv superposed relation), results in a paper that is so entirely different in appearance and structure from calendered creped paper that no one could fail to distinguish them. The crushing of corrugated creped paper also results in an entirely different type of sheet, since folds or pleats are produced which are large compared to the creping wrinkles and the latter are not smoothed down; the caliper thickness is much greater than that of the pre-corrugated creped paper; and a relatively fiat and smooth type of surface does not result. The calendered creped paper to which we refer consists of a single thickness of paper, free from folds or pleats, and in this sense is flat paper.

The severe calendering of porous creped paper to which previous reference has been made will now be explained in more detail with particular reference to the alteration in structure of the paper which is produced.

The creping of porous paper produces a multiplicity of fine wrinkles per inch and the over-all length of the web is materially reduced. The fibers are bent at the creases and are set so as to hold the crepe in the dry paper. The creping operation also opens up the fiber structure so that the body of the paper is less dense and hence is made more porous and absorptive. The degree of creping and stretch can be reduced by pulling out the paper during the making operation. Thus from the same paper and creping operation there can be produced a variety of creped papers having different degrees of residual stretch. This pulling out of creped paper causes elongation equal to the reduction in stretch value and decreases the caliper thickness (due to lessening the height of the ridges), but it does not increase the density of the body structure. The crepe wrinkles remain clear cut and plainly evident to sight and touch.

When porous creped paper is severely calendered by being passed through the steel or other rigid rolls of a paper calender, it is subjected to a wedging and squeezing action at the nip which is of short duration. The curved calender roll surfaces move down into and then up from the paper, as it goes through the nip, and the fiber body structure has room to yield or give at each side of the center of the nip. However, the radius of curvature of the calender rolls is too large to permit of the nip surfaces getting into the valleys of the creping in such a way as to cause elongation of the paper and decreping. The effeet is to roll down the creping wrinkles so that the. paper surface becomes relatively flat, and has a much softer and smoother feel to the touch of a finger rubbed across the flattened remains of the wrinkles. The change in visual appearance is striking. In addition, the squeezing action of the rigid rolls compresses and compacts the body structure of the paper. These two types of compaction (reduction of wrinkles and com pression of body structure) are both involved in what we refer to as severe calendering of the creped paper. is obtained than would result from merely pull-- his out the creped paper; and the density of the paper is increased, thereby reducing porosity. The paper is not substantially elongated by the calender squeezing action.

As an illustration, a 27 pound per ream creped kraft paper (similar to towelling paper), having a'normal caliper thickness of 7.0 mils and a stretch of 12.5%, was calendered to a caliper thickness of 3.8 mils with an. elongation of only 1.0%, and a reduction in stretch to 10.0%. The

of airporosity on a Gurley densometer. In this measurement two thicknesses of paper are;

Thus a greater caliper reduction absence;

clamped-acrossa: circular orifice (l f diameter) and AGO-cc. of air. is forced through .under con-r stant pressure, and the required time is measured. The greater thetime required, the more dense and less porous is the paper. In thiscase the density value was increased from 4.8 seconds to 9.6 seconds, thus demonstrating the substantial compaction of the body structure; The caliper. thickness of calendered creped paper was ap,-- proximately the same as that of the original paper prior to creping, but the density was only about half as great. The original paperprior to. creping would elongate about 1 to 2% when pulled to. the rupture point; but-.thisrepresents a yielding or-pulling apart of the fiber structure preliminary to breaking, as distinguished from the substantial stretch of creped paper obtained.

by-handpulling with a force insufiicient to cause.

incipient rupture.

Aiurther demonstration .of the change in phys.-- ical propertiesproduoed by severe calendering of porous creped paper, can be easily made by subjecting, samples ofv non-calendered and calendered creped papers to successively increased pull stresses ina tensile testing machine, and measuring the required tensile stress for each of various percentages of elongation (up to the breaking, point) and the caliper thickness. at.

each step; An experiment of this type on absorbent creped kraft paper having a caliper thickness. of. 8-9 mils, reduced to 5 mils by calendering, showed that the calendered paper maintained the -.'same thickness. even-to the breaking point,

whereasthe non-calendered paper underwent a caliperreduction proportionate to elongation so thatat thebreaking point the caliper was reduced told-mils; Thiaexperiment also demonstrated the increase in body density of .the calendered. paper, since .the'ultimate. caliper of the stretched calendered'paper' was 5 milsas against 6 mils for-,the .non calendered paper; This'same experiment also demonstrated the.change in the.

stress-strain. characteristic.v A somewhat 1 greater pullingjiorce- (stress) was required to produce.

theesame-degr'ee of elongation of the calendered paper as compared with the non calendered. paper. For example, a pull of 8.4 lbs. per inch width was required toproduce a 5% elongation of theicalendered paper, as comparedto: 6.9 lbs. for: the non-calenderedz-paper.

Calendering, of the: porous. creped paper? in? volves: a. reversiblewchange of structural state. Thecsqueezing action bends-and curls the fibers,- and compacts them, but within limits of elastic regainorreversion; Thus the fibersare not cut;.br0ke1r10r" damaged and this makes possible therproduction of finished masking tape having substantially ashigh stretch and tensile strength valuesiaswould be obtained. if the creped paper had? not been calendered;v despite thefact that these: values are. substantially lowered by the calendering in respect to the paper as it exists priontov coating with the pressure-sensitive ad-- hesive;: This was an unexpected discovery on our:part and isan important featureof our invention as applied to; the manufacture-of maskingatapes and the like.

The reversible nature of the-calendaring action can be easily demonstrated by moisteninga sheet ofithe calendered creped paper and air. drying,

which causes it torevertto a normal creped paper.

substantially identical in appearance, ,feel, and. physical properties, to;the original creped paper (not calendered).

01: course it is: possible; to 1- load. the calender.

so asto'; exert so :greatiapressure'asvwouldibreaki ingjand impregnating compositions whichwould resultin water absorption by. therpaper body;

fibers; Organic solvents are suitable. Also,.the.

calendered creped paper should be' unifie'd bee forereversion takes placeas thBTIBSHltOf unduly long;- storage.

Thei calendered creped paper is stabilized by. coating-orv impregnating With' arbinder material whichbonds the fibers-together; This unification treatment" also increases the tensile strengths The unifying composition should be of a stretchable-kind, such as a rubbery'base materiaLin. order topermitiof'stretchingzthe paper. It should preferably be of a' kind. that adheres: or bondsrto cellulose" fibers in order to cement the: fibers; as. distinguished from merely encasing; them,v in or'- der to obtain a'highjncrease'in tensile strength.

An illustration of a commercially employed pregnation treatment for porous creped." paper used for masking tape: backings,. is the satura-- tion: of the paper with. a: solution arvolatile. hydrocarboniso'lv'ent of. a fluxed. blend of brokendown rubber (natural. or synthetic), a 00m? patible tack-producing" resin (such as. rosin) adaptedto increase adhesion: of theimpregnant' to the paper fibers, and a reinforcingpigment (such as zinc oxide). Following impregnation,v the: paper is heated. in an oven to: remove the. solvent. This same treatment can: be. usedfor unifying calenderedicreped paper; While calendering decreaseszth'e porosity of. the paper,.yetz the decrease is not sufficient to prevent adequate penetration of theiimpregnant-solids. Asv previously mentioned, calendering makes it possible to employ a substantially smaller weight of impregnant solids per unit area and yet. obtain a finishedLmasking tape having'tlie same tensile. strength. For further details on the aboveemen tioned type of rubber resin' impregnationtreat ment', see Drew Patent NOT 2,236,527 (issue-dv April 1, 1941), and Kellgren Patent No..2,4-l0,'078

(issued October 29,. 1946);, andthe Example" set forth hereinafter;

In making maskingtape', the thus unified paper" may be'given a thin coatin of .shellac on the back side to facilitate unwinding of the tape from rolls; and is coated on the face side with a pressuresensitiveadhesive, such as the rubber-resin type; which is aggressively tacky, stretchable; waterproof, and more highly cohesive than adhesive.

The adhesive sheeting is then slit and wound into tioned reversion effect in the paper (especially when the tape is wound in rolls). The back surface of the backing sheet may be provided with a low-adhesion film coating which presents a back. surface toward. which the pressure-sensitive ad- .hesive hasa lowaflinity so that amuch'reduced;

7 force is exerted in unwinding the tape from a roll. Examples of such low adhesion compounds are the cellulose tristerate, tripalmitate and trilaurate esters.

The invention is not limited to the ordinary type of creped paper wherein the wrinkles extend transversely across the web. X-creped papers may be used, wherein a combination of creping wrinkles running crosswise of each other results in a two-way stretch characteristic (1. e. the creped paper web has a crepe stretch both lengthwise and crosswise).

Creped tissue papers find application in certain types of adhesive tape constructions, particularly tissue papers made from long hemp or rope fibers, and these may advantageously be calendered.

Further examples of rubbery, stretchable, waterproof unifying compositions suitable for coating and impregnation, which bond to the cellulose fibers to provide added tensile strength, and which may be applied as a solution in a volatile organic solvent, are the polyacrylate elastomers, such as methyl acrylate polymers, ethyl acrylate polymers, and copolymers of methyl and ethyl acrylates. These can be dissolved in volatile solvents such as acetone and ethylene dichloride.

The calendered creped paper may be directly coated or impregnated with a pressure-sensitive tape adhesive of the rubbery base or rubber-resin type. Thus if such adhesive, in solution in a volatile organic solvent, is applied to both faces of a calendered creped paper, there will be a certain degree of penetration of the paper structure (depending on the type of solution and technique employed), resulting in fiber-unification, and a double-coated adhesive sheet will be obtained which is normally tacky on both sides. A highly porous calendered creped tissue paper can readily be saturated throughout its thickness with the adhesive, and will serve as a stretchable reinforcement within the body of a stretchable pressuresensitive adhesive layer. The adhesive-paper sheet, normally tacky on both sides, can be laminated to a stretchable non-fibrous film backing (such as a film of rubber hydrochloride) for making stretchable film-backed adhesive tapes which have increased tear-resistance and tensile strength on account of the fibrous paper reinforcement.

The porous creped paper may be fully calendered in a single step or may be subjected to suecessive calendering operations. In a single step procedure, a single pair of calender rolls is used, the paper passing around one roll and then through the nip. The pressure loading of the rolls is adjusted to result in suificient squeezing of the creped paper to produce the desired caliper reduction (at least one-fifth) and compaction of the body structure of the paper. The rolls are driven and the paper moves through the nip with the same velocity as the roll surfaces. The calendered paper is drawn to the windup roll without using sufiicient pull tension to draw out the paper. A calender stack having three or more rolls may be used for calendering the paper in two or more successive steps, the final nip pressure being such as to produce the desired ultimate caliper reduction.

The present calendering procedure is clearly distinguishable from the use or draw rolls having resilient surfaces of soft rubber or felt, which are employed in the manufacture of creped paper for drawing out the creping of creped paper to reduce the degree of creping and stretch and to even the creped paper. The paper web is elongated in proportion to the reduction in stretch, and there is no rolling down of the wrinkles and compaction of the body structure. The product is still creped paper, and requires calendering as herein described to result in the distinctive calendered creped paper to which we refer. Likewise the present effect is not produced by the use of evening rolls as employed in the manufacture of creped paper.

It is not necessary to employ hot calendering, i. e. the calender rolls may be unheated or merely warm (say F.). The calendering operation is a dry one, i. e. the paper is not wetted or soaked so as to contain free water like water in a sponge. A creped paper having the degree of dryness resulting from storage under normal room conditions in average humidity air, can be calendered to produce the desired change and will hold the change for a sufiicient number of days, without substantial reversion under storage, to avoid the necessity of immediate coating or impregnation. However, a better and more permanent set can be secured by humidifying the creped paper, especially if it has been stored in relatively dry air, so as to increase the moisture content. This can be done by passing the creped paper through a high-humidity chamber, or by steaming it, prior to calendering. If the moisture content is substantially higher than that which the calendered paper would have upon storage, the calender rolls can be heated sufficiently to dry the paper down to an average moisture content.

The present calendering procedure is, as previously made clear, a dry operation performed on porous creped paper in the absence of free water. The squeezing of a wet creped paper which has been impregnated in an aqueous glue bath (as in Drew patents Nos. 1,760,820 and 1,814,132), so as to squeeze out the surplus impregnant and water, is not a calendering operation and is not the equivalent of the present method, not only because the desired structural alteration and caliper reduction are not obtained, but also because the presence of free water causes a reversion of structure so as to restore the original creping.

Example This example illustrates a complete procedure for preparing unified calendered creped paper, useful for making adhesive masking tapes and for other purposes, and the beneficial results of the calendering operation in the manufacture of masking tape.

The paper used was a regular commercial creped paper that has been commercially used in the manufacturing of masking tapes. It was an absorbent creped paper (made from a bleached and purified kraft paper) having a ream weight of 27 lbs. (weight of 320 square yards), a caliper thickness of 7.0 mils (0.0070 in.), a stretch of 12.5%, and a lengthwise tensile strength of 4.6 lbs, per inch width. It had been stored in stock rolls at room temperature, at a relative humidity of about 35-50%, and was not moistened or humidified prior to calendering.

The laboratory calender employed had a pair of polished steel rolls, diameter 6 in., both positively driven, and the roll speed was 12 R. P. M. The nip spacing was adjustable to provide positive spacing of desired magnitude. The rolls were not heated. Three lots of creped paper were calendered at different pressures to secure difierent degrees of caliper reduction, and are designated hereinafter as I, II and III. One lot "aessg rso lengthwise tensile ian'd stretch values of :the im- ;pregnated papers.

4 Imp ess v V B er 'Galiper "nant Tensile -Stretch, (mils) Weight obs.) remain The impregnation increased the stretch and tensile*ValuesineachcaSe.

The "impregnated "papers were given a backsizing coating of shellac and were then coated on the face iside :a :solution of pressure- -sensitive tapeadhesive having the iormula:

Parts by -weight Natural ru e 100 Zinc oxide 50 .Anti-oxidant (condensation'product of acetone and=a-niline l (The above -werecombined on a rubber "mill, with-millingfor 20 minutes, andthen were *churn-mixed with the following to provide the adhesive solution?) Wood rosin 50 Lanolin 12 Ethyl alcohol (denatured) 7 Heptane (volatile solvent) 410 The sheetings were dried in an oven to remove the solvent from the adhesive coatings, and were slit and wound in rolls.

The finished tapes had the following measured properties:

Caliper Tensile g f gg ll. 5 l9. 5 l8. 1 7. 5 20. 5 18. 8 6. 5 21. 0 16.9 5. 5 21. 0 17. 2

The above table shows that a substantial reduction in caliper of the finished tape was obtained without sacrifice of tensile and stretch values, notwithstanding that the unified calendered creped papers had materially lower tensile and stretch values, and that a substantial saving of the irnpregnant weight has been obtained.

Ihe increase in tensile strength of the adhesive coated unified papers cannot be attributed to a mere additive eiTect, since the pressure-sensitive adhesive coatings, while very stretchable, have only a slight tensile strength per so (as measured in the form of free films thereof). There is some kind of combination eifect, the nature of which is not fully understood, that results when the adhesive is coated on the unified paper, and causes the substantial increase in tensile strength of the "product. It will be noted thatthestretch values have also been substantially increased'as the result of coating with the pressure-sensitive adhesive.

Experiments made with unified calendered creped paper of the II type (see first table), using different coating weights of pressure-sensitive adhesive, have shown that 13-14 grains per 24 sq. in. '(dry weight) results in a masking tape having 'an'adhesion performance that satisfactorily duplicates that obtained by a 17-19 grain coating on the unified non-calendered '(control) creped paper. This demonstrates the substantialsaving in adhesive that can be obtained.

A description willnowbe' given "of the impregnation treatment employed for unifying the papers of this example.

The compounding formula of "ingredients '(other than the vulcanizing'a'gent) wasras follows:

Parts "by weight Natural rubber Zin'c oxide 100 Wood rosin Beta naphthol '1 Oleum spirits (volatile petroleum hydrocarbon solvent of 306-424 F.-boiling range 210 The rubber and zinc ox ide were -mixe'd on "a rubber mill 'f0r'3'0 minutes, thereby thoroughly dispersing the'latter in therubbe'r. "Furthercomtogether with 4=parts-dfthe rosin, werea'dde'd;

after lO minutes of mixing, 1 9 additional parts ofth-e'rosin were added and mixing was continued for 8 hours-with 604% steam in 'the' j-acket. "The balance of the *rosin was then gradually added during a period of 1 hour with continued mixing. The steam was then turned oh and cooling water was sent through the jacket. The beta naphthol was added and then the oleum spirits solvent was added over a period of one-half hour, with continued mixing. The batch was completed, and dumped from the mixer, 10 hours from the start.

Shortly before use for saturating the paper, there was added to the solution a rubber vulcanizing agent, namely, 3.5 parts of dipentamethylene-thiuram-tetrasulfide. This is a selfvulcanizing organic accelerator which releases nascent sulfur.

After saturation treatment with this solution, the impregnated paper was oven-dried for 5 hours at 212 F. to remove the solvent and vulcanize the rubber.

Use can also be made of towelling-type absorbent creped papers which are of the wetstrength kind, due toinclusion of a minute proportion ofasuitable synthetic resin material such as urea-formaldehyde or melamine, which greatly increases the wet strength of the paper but without materially changing its porosity and absorbency. An illustration is -a creped paper having a ream weight of 30 lbs., an initial stretch value of 12%, and an initial caliper of 8 to 9 mils, which is reduced to 5 mils by the calendering operation.

Creped papers having unusually high stretch values can be employed; An illustration is a creped paper having a ream weight of 30 lbs., an initial stretch value of 20%, and an initial caliper of 8 to 9 mils, which is reduced to 5 mils by the able creped paper, in the absence of free water,

so as to roll down the creping wrinkles, decrease the caliper thickness by at least one-fifth, and substantially increase the density of the body structure, the calendered paper being capable of reversion; and thereafter, without allowing reversion to the pre-calendered creped state, unifying the paper by application of a flexible and stretchable fiber-bonding composition which prevents reversion, the unified paper having a stretch value of at least 10%.

2. The method of making a new and useful type of stretchable unified paper which comprises severely calendering a flat, porous and stretchable creped paper, in the absence of free water, so as to roll down the creping wrinkles, decrease the caliper thickness by at least one-fifth, and substantially increase the density of the body structure, the calendered paper being capable of reversion; and thereafter, without allowing reversion to the pre-calendered creped state, unifying the paper by application of a waterinsoluble flexible and stretchable fiber-bonding composition which results in the unified paper having substantially higher stretch and tensile strength values than does the pre-unified calendered paper, the unified paper having a stretch value of at least 10%.

3. The method of making a new and useful type of stretchable unified paper which comprises severely calendering a flat, porous and stretchable creped paper, in the absence of free water, so as to roll down the creping wrinkles, decrease the caliper thickness by at least one-fifth, and

substantially increase the density of the body structure, the calendered paper being capable of reversion; and thereafter, without allowing reversion to the pre-calendered creped state, uni fying the paper by impregnating it substantially throughout its thickness with a stretchable rubbery base unifying composition containing a resin adapted to increase the cementing action upon the fibers of the paper, deposited from solution in an organic solvent, the unified paper having a stretch value of at least 10%.

4. The method of making a new and useful type of stretchable unified paper which comprises severely calendering a flat, porous and stretchable creped paper, in the absence of free water, so as to roll down the creping wrinkles, decrease the caliper thickness by at least one-fifth, and substantially increase the density of the body structure, the calendered paper being capable of reversion; and thereafter, without allowing reversion to the pre-calendered creped state, unifying the paper by impregnating it substantially throughout its thickness with a water-insoluble flexible and stretchable fiber-bonding organic polymer, deposited from solution in an organic solvent, the unified paper having a stretch value of at least 10%.

WALDO KELLGREN. JOHN W. MARSCHALL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Hayward July 9, 1940 

1. THE METHOD OF MAKING A NEW AND USEFUL TYPE OF STRETCHABLE UNIFIED PAPER WHICH COMPRISES SEVERELY CALENDERING A FLAT, POROUS AND STRETCHABLE CREPED PAPER, IN THE ABSENCE OF FREE WATER, SO AS TO ROLL DOWN THE CREPING WRINKLES, DECREASE THE CALIPER THICKNESS BY AT LEAST ONE-FIFTH, AND SUBSTANTIALLY INCREASE THE DENSITY OF THE BODY STRUCTURE, THE CALENDERED PAPER BEING CAPABLE OF REVERSION; AND THEREAFTER, WITHOUT ALLOWING REVERSION TO THE PRE-CALENDERED CREPED STATE, UNIFYING THE PAPER BY APPLICATION OF A FLEXIBLE AND STRETCHABLE FIBER-BONDING COMPOSITION WHICH PREVENTS REVERSION, THE UNIFIED PAPER HAVING A STRETCH VALUE OF AT LEAST 10%. 